Positive displacement rotary gas meters generally consist of a counter module and a gas meter pressure body. The gas meter pressure body converts gas flow into rotational motion via two solid impellers, which sweep out a known volume on each rotation. The impellers are precisely linked together using timing gears such that they can rotate in a desired configuration. The clearances between the impellers can be tight in order to minimize the amount of gas that leaks around the edges of the impellers. As a result of their fixed orientation and swept volumes, when the impellers are driven by a gas flow, the impellers can allow a set volume of gas to pass through the meter on each rotation. Accordingly, the quantity of gas passing through the meter can be calculated by counting the number of rotations of at least one of the impellers. Counting impeller rotations is commonly done by configuring the gas meter such that a mechanical or electronic counter module can be driven by at least one of the impeller shafts via the use of a magnetic coupling module or direct drive.
However, gas flowing through a gas meter may contain dirt, rust particles and other contaminants that can interfere with the proper operation of the meter. Some contaminants may pass through the gas meter causing very little damage to the meter. However, some contaminants can interfere with the operation of the gas meter. For example, dirt may become trapped within the gas meter causing scratching and abrasions between impellers, or between the impellers and the chamber walls. Damage to the impellers or chamber walls can lead to increased gas leakage and reduced counting accuracy. In addition to damaging gas meter components, some contaminants may be of sufficient size and strength to interfere with the impellers and prevent their rotation. Contaminants that are larger than the clearance between the impellers can become wedged between the impellers (or between an impeller and the chamber wall) preventing rotation. Such a condition can be referred to as “seizing”.